1. Field of the Invention
The present invention relates to a microwave heating apparatus that consumes less electric power in a standby state.
2. Description of the Prior Art
FIG. 1 shows an example of the electric circuit diagram of a conventional microwave heating apparatus. This microwave heating apparatus has a plug 1 that is connected to a commercial alternating-current electric power, a step-down transformer 2 for stepping down the voltage of the commercial electric power taken in via the plug 1, a full-wave rectification circuit 3, a smoothing capacitor 4, load circuits RL, a three-terminal voltage regulator 5, and a CPU 6.
The CPU 6, by monitoring the state of a switch 11, detects a door being opened or closed, so that, if the door is opened in the middle of heating operation, the heating operation will be stopped. Moreover, the CPU 6 controls the load circuits RL. The load circuits RL include, for example, various sensors such as an AH (absolute humidity) sensor and a weight sensor, and a relay circuit. The relay circuit controls supply of electric power to a magnetron.
The step-down transformer 2 has a primary coil 2a connected to the plug 1, and has a secondary coil 2b connected to input terminals of the full-wave rectification circuit 3. The smoothing capacitor 4 has one end connected to a high-voltage-side output terminal of the full-wave rectification circuit 3, and has the other end connected to a low-voltage-side output terminal thereof. For example, in Japan, a source voltage of 100 V fed in via the plug 1 is applied, as a voltage AC1, to the primary coil 2a of the step-down transformer 2 so as to induce a stepped-down voltage across the secondary coil 2b.
This voltage appearing across the secondary coil 2b is subjected to full-wave rectification by the full-wave rectification circuit 3 and then to smoothing by the capacitor 4 so as to be made into a direct-current voltage DC1 of about 18 V. This voltage DC1 is used to drive the load circuits RL. The CPU 6 receives a direct-current voltage DC2 that is regulated by the three-terminal voltage regulator 5 so as to have about 5 V. This voltage DC2 is applied, on its high-voltage and low-voltage sides, to the terminals VCC and VSS, respectively, of the CPU 6.
Having a circuit configuration as described above, this microwave heating apparatus has the following disadvantage. Even when the microwave heating apparatus is in a standby state, i.e. even when it is not in operation for cooking, the same voltage as when it is in operation is applied through the step-down transformer 2 to the load circuits 5. As a result, electric power of about 2 W is unnecessarily consumed as iron loss, copper loss, and the like as well as by the load circuits including the various sensors and the relay circuit.
As an improvement to such a microwave heating apparatus that unnecessarily consumes electric power in a standby state, Japanese Laid-Open Patent Application No. H7-217906 proposes a microwave heating apparatus that is so configured as to consume less electric power in a standby state.
FIG. 2 shows the circuit configuration of this microwave heating apparatus. The microwave heating apparatus operates from a power source 51, and has loads 52 such as a heating member and a motor, main door switches 53 and 54 that are turned off or on as a door is opened or closed, a monitor switch 55, a door switch 56 that is turned on or off as the door is opened or closed, and relay switches 57 and 58.
The microwave heating apparatus also has a CPU 61 that serves as a data processing device, and a power supply circuit 62 for producing direct-current electric power on which the CPU 61 and other circuits operate. The microwave heating apparatus further has an output circuit 63 for driving the above-mentioned relay switches 57 and 58, a key input circuit 64 for allowing setting of a cooking program for heating operation and entry of an instruction for starting heating operation, and a door switch detection circuit 65 for detecting the open/closed state of the door; these three circuits are driven and controlled under the control of the CPU 61.
FIG. 2 shows a state in which the door is closed. In this state, the main door switches 53 and 54 are on, the door switch 56 is off, and the monitor switch 55 is in the terminal "b" position. Moreover, the relay switches 57 and 58 are off, and thus no electric power is supplied to the loads 52, the CPU 61, or other circuits.
When the door is opened from this state, the main switches 53 and 54 are turned off, and the monitor switch 55 is brought into the terminal "a" position. Moreover, the door switch 56 is turned on, and thus electric power starts being supplied only to the CPU 61. The CPU 61, when it starts receiving electric power through the power supply circuit 62, first turns on the relay switch 57 through the output circuit 63. Accordingly, the CPU 61 continues receiving electric power even when the door is closed next time and the door switch 56 is turned off.
The CPU 61, at the same time as it turns on the relay switch 57, sets a period of time to be counted by a timer, so that, when the once opened door is closed again, the timer starts counting the period of time. Unless, in the meantime, a key input signal is obtained from the key input circuit 64 or a door open/close signal is obtained from the door switch detection circuit 65, the CPU 61 turns off the relay switch 57 when the timer finishes counting the period of time. Accordingly, when no heating operation is taking place, supply of electric power to the CPU 61 is shut off securely, and thus no electric power is consumed in a standby state.
However, this microwave heating apparatus has its own disadvantage. For example, some of its functions require energizing of the CPU 61 but do not necessarily require heating operation, such as the function of displaying on a display unit, for confirmation, cooking conditions such as heating power and heating time under which heating operation is to be performed or cooking recipe data indicating a cooking method and appropriate heating conditions, or the function of executing a cooking menu selection program. Even to perform such a function, it is necessary to energize the CPU 61 first by performing an extra step of opening the door once.